There are technically infinite copies of digital files, but people still might find long waits at their local library for popular e-books.

“Marketplace” host Kai Ryssdal spoke with Heather Kelly, who wrote about the challenges e-books create for libraries for the Washington Post. E-books are also usually more expensive for libraries than physical books, which can put strain on budgets and librarians themselves.

The way libraries buy books from publishers can be very complicated, according to Kelly, and publishers have their own ways of doing it.

“One of the biggest ones is, ‘We will basically license a book to you for two years,'” Kelly said. “‘You’ll pay us $60 for just that two years, only one person can read a digital file at a time, and if 56 people read it, then the two-year limit is already up.”

That can cause long waits for new and popular titles. In response, some people have figured out how to hack their e-readers to allow them to keep their rented e-books for longer.

Click the audio player above to hear the interview.